The main method for synthesizing acrylic acid uses a reaction of catalytic oxidation of propylene with a mixture containing oxygen. This reaction is generally carried out in the vapor phase, usually in two steps, which may be carried out in two distinct reactors or a single reactor:                the first step carries out the substantially quantitative oxidation of the propylene to an acrolein rich mixture, in which the acrylic acid is a minority component;        the second step completes the conversion of acrolein to acrylic acid.        
The gas mixture issuing from the second oxidation step consists of:                acrylic acids;        light compounds incondensable in the temperature and pressure conditions commonly employed (unconverted nitrogen, oxygen and propylene, propane present in the reactive propylene, carbon monoxide and dioxide formed in small quantities by final oxidation);        light condensable compounds, particularly water, generated by the propylene oxidation reaction, unconverted acrolein, light aldehydes, such as formaldehyde and acetaldehyde, and acetic acid, the main impurity generated in the reaction section;        heavy compounds: furfuraldehyde, benzaldehyde, maleic anhydride, etc.        
A method well described in the literature for synthesizing methacrylic acid by oxidation is identical in principle to that of acrylic acid, except for the reactive substrate (which may be isobutene or tertbutanol), the intermediate oxidation product (methacrolein) and the types of light condensable byproduct compounds (the reaction gas mixture contains, among other compounds, acrylic acid in addition to the light compounds present in the reaction gas of the acrylic acid synthesis method).
The second stage of fabrication consists in recovering the acrylic acid from the hot gas mixture, previously cooled to a temperature of 150-200° C., by introducing this gas at the bottom of an absorption column where it meets a countercurrent flow of absorption solvent, which we shall refer to below by the term “solvent”, introduced at the top of the column, and inside which cooling, condensation, absorption and rectification processes take place simultaneously. In most of the methods described, the solvent employed in this column is water or a high boiling point hydrophobic solvent.
The use of a heavy hydrophobic solvent in this first absorption section, as described in French patent No. 2 146 386 and German patent No. 4 308 087, has advantages over the use of water, the main one being to make the separation of the light compounds far easier, particularly water and acetic acid, which are difficult to remove in the methods with aqueous solution.
On completion of the absorption step, the known methods for recovering (meth)acrylic acid from the solutions of the monomer in the solvent involve a step of removal of the light compounds at the top of a stripping column, designed to send most of these light compounds to the preceding absorption column, where they are removed in the head effluent, as described in the two abovementioned French and German patents. For complete removal of the light compounds, it is necessary to use a distillation column for removing the final traces of these volatile impurities at the top. Other alternatives have been proposed, like the one described in European patent No. 706 986, which consists in removing the final traces of light impurities (tapping) at the top of an upper section of the final acrylic acid distillation column, the pure monomer being recovered in a tapped off side stream. A method is proposed in the French patent application filed today in the name of the present applicant and also having the title “Method for purifying (meth)acrylic acid obtained by oxidation of a gas substrate” in which the conditions of the absorption and stripping sections are selected so as to obtain a stream of raw (meth)acrylic acid stripped of most of these light impurities in a single step, without an additional column or tapping section.
The subsequent steps of the purification method, in the case of the use of a heavy hydrophobic absorption solvent, are designed to quantitatively recover the (meth)acrylic acid, to separate the heavy impurities from the solvent, and to regenerate the solvent for recycling to the absorption step. In fact, the need to recover and recycle the solvent in a sort of “loop” is necessary for economic reasons and to avoid pollutant releases, making it necessary to separate and remove the compounds heavier than acrylic acid, in order to prevent their holdup in this solvent loop. For this purpose, a sufficient quantity of these heavy compounds must be purged, at least equal to the quantity fed to the first absorption section.
Among the compounds heavier than acrylic acid, mention can be made of the impurities generated in the reaction step, and also the polymerization inhibitors introduced at each step of the purification method. In fact, since (meth)acrylic acid is a sensitive product readily prone to a polymerization process favored by the temperature, polymerization inhibitors are introduced into the purification equipment in order to avoid this process.
Numerous polymerization inhibitors are conventionally mentioned in (meth)acrylic acid purification methods. Among them, mention can be made of phenolic compounds, such as hydroquinone or methylether of hydroquinone, phenothiazine and its derivatives, such as methylene blue, quinones such as benzoquinone, manganese salts, such as manganese acetate, metal thiocarbamates such as copper salts of dithiocarbamic acid, like copper dibutyldithiocarbamate, N-oxyl compounds, including 4-hydroxy-2,2,6,6-tetramethyl-piperidinoxyl, amine compounds, such as derivatives of paraphenylene diamine, compounds with a nitroso group such as N-nitrosophenyl hydroxylamine, and ammonium salts of N-nitrosophenyl hydroxylamine.
The vast majority of polymerization inhibitors are heavier compounds than (meth)acrylic acid, and are therefore not entrained with the (meth)acrylic acid during distillation processes. Thus, these inhibitors are generally introduced at all points of the equipment (column heads, condensers, etc.) which may be the seat of a liquid-vapor equilibrium leading to the condensation of (meth)acrylic acid rich streams. The efficiency of these polymerization inhibitors, used alone or in mixtures, is generally increased if they are used in combination with the introduction of oxygen or a gas stream containing oxygen at the bottom of the column.
The polymerization inhibitors are introduced pure, if liquid, or in solution in a solvent, which is selected advantageously from the absorption solvent or (meth)acrylic acid.
The heavy compounds accumulated in the method also consist of impurities originating from side reactions in the acrylic acid synthesis step by oxidation of propylene or propane, or in the methacrylic acid synthesis step by oxidation of isobutene or isobutane. Another category of heavy impurities is that resulting from degradation reactions during the purification steps. The use of heavy hydrophobic absorption solvents with significantly higher boiling points than that of the (meth)acrylic acid requires the application of high temperature levels, particularly at the column bottom. These high temperature levels promote the degradation of the components present in the stream, for example of the solvent and the polymerization inhibitors, and accordingly demand high energy consumption, to heat the stream to boiling. The thermal levels reached by these streams poor in light compounds may be limited to a certain degree by operating the equipment under reduced pressure, but the requirements of the condensation temperature at the top of the column, which cannot be too low at the risk of generating excessive cooling costs, of column size, of condenser and vacuum generation system, which increase proportionally with the vacuum level, mean that temperatures higher than 180° C. are generally obtained in the hottest parts of the equipment. In consequence, it is preferable to limit the number of unit operations involving the boiling of solvent rich streams.
In the case of methods employing heavy hydrophobic solvents, it is necessary to distinguish the following among the heavy compounds:                “intermediate” compounds, with boiling points between that of the (meth)acrylic acid and that of the solvent. Among these compounds are impurities generated during the reaction step, such as maleic anhydride, furfuraldehyde, benzaldehyde, or polymerization inhibitors more volatile than the solvent;        “heavy” compounds which have a higher boiling point than that of the solvent, among which mention can be made of the impurities formed in the method, such as oligomers, derivatives of addition to the double bond of the (meth)acrylic acid, polymers, degradation products of the solvent or the inhibitors, and the polymerization inhibitors less volatile than the solvent.        
On completion of the step to remove the residual light compounds, the (meth)acrylic acid is the most often recovered, from its mixture with the heavy solvent containing the heavier impurities, at the top of a distillation column, the less volatile compounds than the monomer being removed in the bottom stream from this column.
The removal of a sufficient quantity of heavier compounds than the solvent, in order to prevent their holdup in the method, can be completed by the use of the purge of a stream enriched with these compounds, obtained at the bottom of a solvent regeneration section, consisting in evaporating or distilling all or part of the solvent stream flowing in a loop in the method. The stream enriched with heavier compounds than the solvent is then removed at the bottom of the evaporator or of the distillation column.
However, the removal of the compounds heavier than the solvent is costly in terms of energy and is inevitably accompanied by equally costly loss of solvent, which is poorly separated from the heavier compounds, and of the polymerization inhibitors. In consequence, it is economically advantageous to limit the quantity of stream concerned by the purging of the heavy impurities.
As described in the abovementioned French patent No. 2 146 386, it is possible to separate the intermediate compounds at the bottom of a water scrubbing column, or at the top of a distillation column.
In the case of extraction using water, the method has the disadvantage of generating an additional aqueous stream laden with pollutant compounds which it is costly to treat before release, and furthermore, this method is unsuitable for removing all the heavy impurities, particularly those that are relatively nonpolar. This results in the progressive holdup of these intermediate compounds in the solvent loop, inevitably leading to their entrainment in the distilled (meth)acrylic acid.
In the other case of separation by distillation, the method also suffers from substantial disadvantages. In purification methods using absorption by heavy hydrophobic solvent, the (meth)acrylic acid concentration in the raw skimmed mixture does not exceed 30%, the remainder essentially consisting of solvent. Since the concentrations of heavy intermediate compounds are lower than that of (meth)acrylic acid, these compounds are extremely diluted in the stream consisting essentially of solvent obtained at the bottom of a (meth)acrylic acid distillation column. In consequence, the separation of small quantities of intermediate compounds in a distillation column fed with a bottom stream from the preceding distillation column, as described in French patent No. 2 146 386, requires treating a very large stream of highly diluted mixture. The drawbacks of such a method are a high loss of solvent entrained at the top of the column, costly energy consumption to heat a large quantity of heavy solvent to boiling, and considerably larger equipment size, essentially the column bottom and boiler, implying higher investment costs. Furthermore, such a method does not permit the efficient recovery of the stabilizers that are heavier or lighter than the solvent, for recycling to the columns located upstream of the method.
An improvement to the process of removing heavy intermediate impurities has been described in French patent No. 2 736 912, which claims a purification section consisting in distilling pure acrylic acid at the top of a first column, allowing little monomer to pass at the bottom, and sending the column bottom stream containing the heavy intermediate impurities to feed another column, in tapping off a side stream rich in intermediate compounds, and in recycling the acrylic acid rich column head stream to the preceding column.
This method has the disadvantage of further generating a loss of solvent and polymerization inhibitors.